1. Field of the Invention
The present invention relates to a multi-pulse HVDC system which is capable of minimizing the number of thyristors by providing a thyristor auxiliary circuit in a conventional 12-pulse thyristor HVDC(High Voltage Direct Current), not using an additional stack of a thyristor and which is implemented based on an auxiliary circuit having a multi-pulse thyristor HVDC system like 24-pulse/36-pulse/48-pulse converters.
2. Description of the Background Art
An AC voltage and current outputted from a generator is a low current-large current. When transmitting power, the voltage and current are transmitted in a long distance in a low current type, maintaining a high voltage using a high voltage or ultra-high voltage transformer due to a loss in a conductor by a large current. However, in the above AC power transmission, there is a certain limit in a long distance transmission due to a capacitive load between a transmission cable and ground and an inductance existing in a transmission cable.
As a mess storage thyristor is developed to convert the AC into DC, it becomes possible to transmits a direct current which was a problem in the art. Since the direct current transmission method is implemented based on a thyristor which is capable of performing a phase control, a harmonic problem occurs. Namely, a harmonic component occurs in connection with the number of pulses of a converter in an input current and output voltage due to a switching operation of a thyristor convertor. Assuming that the number of pulses of a converter is P, a harmonic of Pn±1(n represents an integer) occurs in an input current, end a harmonic of Pn occurs in an output voltage.
Therefore, as a method for decreasing harmonic, there is known a method for increasing the number of pulses of a thyristor, a methods for inserting a harmonic liter in the side of AC, and a method for removing a harmonic problem using a device having an arc extinguishing function like IGBT(insulator Gate Bipola Transistor) of GTO(Gate Turn Off Thyristor). Among the above methods, the self-arc extinguishing HVDC system implemented using IGBT end GTO has many problems for actually using the same since a technical review is not ended.
In addition, the above method for increasing the number of pulses of a thyristor and the method for additionally inserting a harmonic filter in the side of the AC are generally implemented in such a manner that the pulses of the thyristor is 12, and a harmonic filter is inserted in the side of the AC in consideration with an economical reason. If the number of pulses of a thyristor exceeds 12 pulses, since the harmonic is decreased, the capacity of the harmonic filter is advantageously decreased, but an economical burden problem occurs because a thyristor stack must be additionally provided.
FIG. 1 is a circuit diagram illustrating a HVDC using a conventional thyristor. As shown therein, since the HVDC system uses a natural arc extinguishing device like a thyristor, a 6-pulse thyristor stack like thyristor converters 12, 13 are basically used. Additionally, a 6-pulse thyristor stack of a Δ winding transformer which has a certain degree behind by 30° is connected to a Y-winding transformer in series for thereby increasing the number of pulses of a HVDC system and decreasing a voltage harmonic in the side of DC.
In the above conventional HVDC system, it is needed to connect a lower order harmonic filter 17 capable of decreasing a lower order harmonic and a higher order harmonic filter 18 to the side of AC due to a harmonic component. In addition, a Y-connection is performed in a primary side using a Δ-Δ-Y transformer 11 without using a Y-Δ transformer, and a Y-Δ transformer is used in a secondary side. Since the HVDC system of a thyristor type is a constant current system, a reactor 15 is additionally needed in a DC output terminal.
FIG. 2 is a block diagram illustrating a controller of a conventional 12-pulse thyristor HVDC system, which includes an instruction unit 21, a detector 27, a noise filter 26 for removing noises of a signal detected, and an adder 22 for comparing an instruction signal and a detected signal and generating an error signal. The error signal is inputted into a comparator 25 through a proportional controller(K) capable of amplifying an error signal and an integrator 24 which integrates an error signal.
In addition, the thyristor HVDC system operates based on a basic principle that a thyristor is driven with a regular interval signal for decreasing any effects of a harmonic. The output signal of a filtering circuit 30 has a constant DC value in such a manner that the portions synchronized to a frequency of a system corresponds to a ramp generator 28, and an output of the ramp generator 28 is a digital saw tooth wave coinciding with the frequency of the system, and the above value is inputted into a filtering circuit 1/(1+ST) 30 through a sampling/holder(S/H) 29.
When the above value is compared with a saw tooth wave from the ramp generator 28 by the adder 22′, an output value of the adder 22′ becomes a reverse saw tooth waveform. The above reverse saw tooth waveform is compared with en error value from the integrator (1/s) 24 by the comparator 25, so that a firing pulse of a thyristor occurs at a point where the size of the reverse saw tooth wave meets with the error value.
At this time, in the case that the generated pulse fires 12 thyristors, the pulse becomes 12 pulses, and in the case that the generated pulse fires 6 thyristors, the pulse becomes 6 pulses. In the case that the number of pulses of the thyristor is increased in the method of FIG. 1, the stacks of the thyristors are connected in series.
FIG. 3 is a circuit diagram illustrating a conventional 24-pulse thyristor HVDC system. In the 24-pulse thyristor HVDC system, a 12-pulse thyristor is additionally connected to the 12-pulse thyristor circuit of FIG. 1 in series. Namely, the above construction is implemented in such a manner that the 12-pulse circuit of the Δ-Y-Δ transformer 11′ and the 12-pulse circuit of the Δ-Δ-Y transformer 11 are combined. The above method has disadvantages in an economical basis as the number of the thyristors is increased. In addition, the converters 12′ 13′ of the 6-pulse thyristor stack is connected to the Δ-Y-Δ transformer 11′, and the converters 12, 13 of the 6-pulse thyristor stack is connected with the Δ-Δ-Y transformer 11.
FIG. 4 is a circuit diagram illustrating a 24-pulse HVDC system using a conventional auxiliary circuit. The above circuit has an operation characteristic of the operation system of the 24-pulse thyristor HVDC system of FIG. 3 and is capable of enhancing an efficiency of the system using the decreased number of the thyristor devices.
Namely, as shown in FIG. 1, in the operation principle of the 24-pulse HVDC system using an auxiliary circuit, there is provided a 24-pulse thyristor HVDC system having an additional auxiliary circuit having a characteristic that the 24-pulse operation is implemented by decreasing and increasing the thyristor output current i1 using the auxiliary circuit capable of reversely flowing the current in the 12-pulse circuit as shown in FIG. 1.
Therefore, since the current reverse flow auxiliary circuit is additionally connected to the conventional 12-pulse thyristor HVDC system for thereby performing the operation based on the 24-pulse thyristor HVDC system, it is not economical.